The present invention relates generally to the microwave receiver art, and more specifically concerns microwave receivers useful in instrument landing or other navigation systems.
Instrument landing systems (ILS) for aircraft which use carrier frequencies in the microwave frequency range (i.e., above 3 GHz) are often desirable as opposed to conventional VHF/UHF ILS systems because significantly smaller antenna apertures may be used at the transmitter location. In conventional VHF/UHF ILS, transmitting antennas of reasonable size tend to project beams which are poorly defined and difficult to control, a significant disadvantage in those applications where beams must be tailored to prevent interference from particular structural or topographical monuments adjacent the landing path of the aircraft. The use of microwave frequencies, on the other hand, permits a significant amount of beam control using a reasonably sized antenna aperture. These principles of microwave ILS, with attendant beam shaping, are more fully explained in U.S. Pat. No. 3,798,646, to Buehler, et al. and assigned to the same assignee as the present invention.
The use of microwave carrier frequencies, however, demands that the onboard aircraft receiver be suitable for processing microwave frequencies. Heretofore, this has been accomplished in either one of two ways. In the first approach, a conventional (i.e., UHF/VHF) receiver is converted to handle the microwave frequencies, or, in the second approach, a microwave receiver is specially designed to process the microwave frequencies of the particular ILS system being used. In the first approach, a significant and expensive problem develops because of the extremely high stability of the local oscillator necessary to properly downconvert the microwave carrier frequencies to the frequencies normally processed in the UHF/VHF receiver. Thus, the converted microwave receiver includes an add-on device at its front end which downconverts the received microwave frequencies to the frequencies for which the VHF/UHF receiver is designed.
In a typical UHF/UHF receiver, the carrier frequency for the localizer beam will be 110 MHz, and the carrier frequency for the glide slope will be 330 MHz. Because of rather severe channel bandwidth restrictions in those frequency ranges, and in order to maintain the noise level at a tolerable magnitude, data bandwidths of 40 KHz and 120 KHz are typically provided with respect to the 110 MHz and 330 MHz carrier frequencies, respectively. Since the carrier frequencies in the microwave ILS system are typically above 3 GHz, and in the ILS system used with the receiver of the preferred embodiment are 5.10 and 5.23 GHz, (i.e., C-band microwave frequency) the frequency of the local oscillator which in combination with a mixer downconverts the carrier frequencies to the VHF/UHF carrier frequencies must have a stability which is directly proportional to the data bandwidths of the two carrier frequencies. That is, the narrower the data bandwidth, the greater must be the stability of the local oscillator, so that the down converted signal does not drift out of the bandwidth. To adequately maintain the data bandwidth at 40 KHz the stability of the local oscillator must be on the level of 1 part in 10.sup.7, which is a very high stability requirement and requires an oscillator which is temperature controlled and rather expensive.
The other approach, as noted above, uses a microwave receiver specially designed for processing microwave carrier frequencies. In the microwave frequency range, the channel separation is typically on the order of hundredths of MHz, and therefore, does not require an oscillator with the stability required by receiver using the converted VHF/UHF approach. Such a microwave receiver, however, has heretofore included a local oscillator with a mixer followed by IF amplifier which in turn is connected to conventional detection and amplifying circuits of the ILS indicating equipment in the aircraft. Such an approach, although workable, increases the cost of the on-board system significantly and thus denies the use of ILS capability to many aircraft.
Accordingly, it is a general object of the present invention to overcome those disadvantages of prior art ILS receivers discussed above.
It is another object of the present invention to provide an ILS microwave receiver which is compatible with existing ILS indicating equipment in aircraft.
It is a further object of the present invention to provide such a microwave receiver which does not require the use of a local oscillator or a mixer.
It is another object of the present invention to provide an ILS microwave receiver which is relatively inexpensive to manufacture.